Since the late 1960's, when waterbeds were first introduced, waterbeds have been gaining increasing acceptance. It has been recognized that waterbeds provide dimensions of comfort and bodily support not available from conventional inner spring or foam mattress bedding systems. Waterbeds have also become favored for therapeutic and other user qualities.
The structural design of waterbeds has evolved since their introduction. Waterbed mattresses were initially utilized without outside supports, or else, were disposed in simple wooden frames. It has always been preferable to support the sides of the mattress in a frame to provide support to the mattress itself, thereby preventing excessive strain on the mattress material. However, wooden side rails comprising the early frame designs are hard, and therefore, are uncomfortable to users who bump, lie or sit on them when getting out of, or into, the waterbed. Other prior art waterbed frames comprised hard foam side rails in place of the wooden side rails, the foam side rails being integrally packaged with the mattress into a single unit.
Another variation of frame design is disclosed in U.S. Pat. No. 4,197,602 issued to Johenning in which the frame is formed of foam pads with a pocket therein along the long access thereof, said pocket containing hard wooden support slats disposed therethrough. This design is comfortable to a user and provides reasonable support to the sides of the mattress. However, this frame is rather cumbersome to manufacture in that the side rail pads have to be formed with pockets to hold the slats, and the slats must be installed in said pockets.
Another drawback of early waterbed designs is that they were succeptiable to excessive wave motion in the waterbed mattress, which can make users of the waterbed uncomfortable. A number of solutions to the problem of excess wave motion have been developed, including the hybrid waterbed. In a hybrid waterbed, the frame thereof, generally referred to as a cavity, comprises four side walls of firm urethane foam adhered to a base comprising a sheet of soft urethane foam. The side walls have a cross section in the shape of a right trapezoid, the right angle thereof being defined by the outside face and bottom plane of the side wall, and the inside plane facing the mattress having an angle of approximately 350.degree. to the horizontal. The typical dimensions of a side rail include the top plane being three inches, and the bottom plane being six inches, with the height of the side rail also being six inches. A fabricated cover surrounds the entire frame and the waterbed mattress, the entire assembly generally being a finished, integral piece.
Due to the trapezoidal configuration of the side wall with the base surface being wider than the top surface, any pressure on the top surface, such as when the user sits or lies thereon, tends to cause the side wall to bend outward. This results in excess stress on the bond between the side wall and the base, as well as excess stress on the surrounding cover.
In order to minimize this distortion, the foam used in the fabrication of the side walls is a firm grade foam. The firmness of polyurethane and other foams 25 is graded by the foam industry on a scale known as the indent load deflection scale (hereinafter referred to as "ILD"). On this scale, the lower numbers indicate that the foam is very liable, whereas higher numbers of about 100 above, indicate the foam is stiff.
The side walls of a hybrid waterbed frame are normally produced using a foam with an ILD of about 100. Due to the firmness of the side walls, they do not yield to the same degree as a waterbed mattress, and are therefore more like the wooden side rails discussed above. Thus, these side rails are relatively uncomfortable and cannot be used by typical waterbed users as part of the normal sleeping surface.